The invention relates to electrical stepper motors and more particularly to circuitry for controlling such motors for holding the stepper motor armature speeds substantially uniform without the necessity of utilizing an encoder driven by the stepper motor.
An electrical stepper motor, which is a device designed for positional accuracy, may be used in a high accuracy constant speed mode, such as for driving magnetic disks either of the rigid or flexible type. Some advantages of using an electrical stepper motor in this manner are: relatively low cost, small size, and absolute long term speed accuracy since the stepper motor is a synchronous device. For driving magnetic disks, an electrical stepper motor is most attractive when used in a direct drive configuration, by directly coupling the motor armature through a spindle with the magnetic disk. Start-stop and zoned speed modes of operation are possible. With such a direct drive, belts and pulleys are eliminated, the size of the magnetic disk file machine as a whole is reduced, and reliability is increased. The reduction of the size of the file machine comes about not only because of belt and pulley elimination but also because of a physical motor size reduction. The latter is due to the increased motor efficiency over present A.C. motor pulley belt systems.
A directly coupled arrangement of the electrical stepper motor with a spindle carrying a magnetic disk also has some disadvantages. The torque variation on the motor armature versus the position of the armature gives rise to large instantaneous speed variations (5-10 percent) when using classical digital drive techniques. This problem has been eliminated through the use of a mini step PWM (PULSE WIDTH MODULATION) type motor driver which, in effect, supplies the motor with a current related to armature position. This forces a constant armature torque versus position characteristic and greatly reduces the torque ripple of the armature. Use of this driving technique has resulted in instantaneous speed variations under 0.75 percent peak to peak. Most non direct drive systems for stepper motors cannot approach this tolerance. The other problem to be solved is poor motor spindle armature response to step load variations. For example, the instantaneous torque load transient response is especially poor in an open loop spindle system which does not utilize an encoder driven by the motor armature for feedback.
Using such prior approaches without feedback, when the stepper motor has a step change in load torque applied to it such as when a transducer is applied onto a magnetic disk driven by the motor, the motor responds in a highly underdamped fashion. In a typical flexible magnetic disk direct drive application, the head load mechanism may, for example, apply a 50 gram step load to the disk surface. The motor responds by oscillating about the desired speed for a substantial time, with the speed of the motor gradually leveling off at its proper value; however, the result is that data cannot be read from the magnetic disk for one or more seconds after the magnetic head or transducer has been applied into contact with the disk. In the past, the only effective electrical way to eliminate the problem and maintain fast response has been to add an encoder to the disk spindle to thus make the drive a closed loop system with the encoder providing a feedback. This approach, although it functions well, greatly increases the cost of the machine and increases its size.
Some past approaches have also attempted to isolate the back EMF voltage produced by the motor for providing an error signal usable in lieu of an encoder signal; however, such isolation resulted in complexity of the circuitry and increased cost.